At the start of the Vanguard Action Squad's invasion of The Beast's Forest, Mustard fills the forest with his toxic gas. This almost immediately incapacitates Juzo Honenuki along with several other students in the forest. Itsuka Kendo uses her Quirk to enlarge her hands and she grabs Yui Kodai with them in order to protect her.
Ibara Shiozaki is also knocked unconscious and Tetsutetsu tries to carry her to safety. He receives gas masks from Momo Yaoyorozu and eventually happens upon Itsuka and Yui. The girls equip the masks and Itsuka says they need to retreat back to camp. Tetsutetsu refuses and claims that he's going to fight back. Itsuka reminds her peer that Mandalay told all the students not to engage.
Tetsutetsu points out that Class 1-A has had chances to prove themselves because they've fought back against villains that attacked them. Refusing to run, Tetsutetsu tells Itsuka not to try and stop him because they're supposed to be heroes and it's a hero's duty to fight back against the criminals that threaten them.
Soon after, Mandalay sends out a telepathic message granting the students permission to engage the villains in combat. This excites Tetsutetsu as he runs through the forest with Itsuka following close behind. Itsuka asks Tetsutetsu if he understands how the gas works. He replies that he knows it's dangerous and for her to give him more credit than to ask that.
Itsuka explains that she meant Mandalay didn't talk about the gas in her message and that means it hasn't reached the area of the forest where she is at. She points out that the gas isn't acting naturally because it's dispersing very slowly and not spreading out. Itsuka surmises that the gas is concentrated at one point and that's where they can find the villain controlling it.
Tetsutetsu understands Itsuka and commends her whits. She points out that the further they go into the gas, the thicker the cloud will become. Their masks have a limit to how much they can filter so the duo charges quickly through the gas and heads for the center to try and find the gas user as quickly as possible. Tetsutetsu mentions that he's angry because his classmates were hurt by the gas. Itsuka silently appreciates Tetsutetsu's fierce attitude.
Mustard sits at the center of a large typhoon of his toxic gas. He senses two people moving towards him and surmises that they've discovered his location. Tetsutetsu breaks through the cloud of smoke and confronts the villain. Mustard draws a gun on Tetsutetsu and quickly shoots him in the face. Tetsutetsu survives thanks to his Steel hardening Quirk, but his mask breaks off. Mustard recognizes Tetsutetsu from the U.A. Sports Festival and realizes his Quirk makes him bulletproof. The villain points out that now the battle will be decided by how long Tetsutetsu can hold his breath.
Tetsutetsu is impressed Mustard took out his mask with a single shot. He ponders why Mustard wears a school uniform and surmises they must be similar in age. Tetsutetsu tries to rush the villain, but Mustard shoots the former and sends him flying back. He taunts Tetsutetsu for being too straightforward and says a U.A. Student should be smarter. Mustard turns his gun to Itsuka, who is still hiding in the gas. He shoots but Tetsutetsu jumps in front of the bullet. He begins to strain his Quirk and his head starts to bleed from getting shot.
Tetsutetsu pleads with Itsuka to retreat while Mustard taunts them for their poor strategy. Mustard explains that he can not only produce and manipulate the gas, but he can read his opponents' movements through the gas's fluctuations. The villain disappears into the gas and Tetsutetsu rushes to find him. Mustard suddenly appears next to Tetsutetsu and shoots him in the head again. Tetsutetsu falls hard on the ground and Mustard points out that his steel is weakening. Mustard asks if it's due to iron fatigue or lack of oxygen before shooting his opponent again.
Mustard berates Tetsutetsu for having a simple one-note Quirk and shoots his last bullet at him. The gas user reloads and gets angry because people fawn over weak fighters like Tetsutetsu just because he went to U.A. High School. Mustard kicks Tetsutetsu and says that attention is what's wrong with the world. Unable to recover, Tetsutetsu takes a few more shots as his eyesight starts to fade and his stamina falls.
Itsuka demands Mustard stop and rushes him from behind. The gas user sidesteps her and reminds his opponent that he can read her movements because of the gas. Itsuka activates her Quirk, enlarging her fists to hit Mustard by surprise. Mustard berates her Quirk and disappears into the gas. Itsuka says that she can win as long as she uses her Quirk correctly. She enlarges both her hands and waves them around to dissipate the gas surrounding them. Itsuka points out that carrying a gun and wearing a gas mask makes it obvious that Mustard knows he can't win a fight on his own merit.
Mustard tries to take aim at Itsuka. She says this fight is for those at U.A. High with simple minds. Mustard doesn't notice Tetsutetsu engaging him in time because the gas got thinner. Itsuka states that U.A. Students with simple minds always go beyond when everyone else is ready to give up. Tetsutetsu cocks his fist back, punches Mustard's face into the ground, completely shatters his gas mask, and knocks him unconscious.
With Mustard defeated, the poison gas disperses completely. Tetsutetsu catches his breath and says the best way to defeat a gas user wearing his own gas mask is to break it. Tetsutetsu falls over in exhaustion next to Mustard and tells him he's going to pay for ruining camp. 
|Forest Training Camp Arc|
|Chapters||70 • 71 • 72 • 73 • 74 • 75 • 76 • 77 • 78 • 79 • 80 • 81 • 82 • 83|
|Volumes||8 • 9 • 10|
|Episodes||39 • 40 • 41 • 42 • 43 • 44 • 45|
|Battles and Events||Class 1-A Swimming Competition • Quirk Training Camp (Class 1-A vs. Pixie-Bob) • Vanguard Action Squad Invasion (Tiger & Mandalay vs. Magne & Spinner • Izuku Midoriya vs. Muscular • Eraser Head & Vlad King vs. Dabi Doubles • Tetsutetsu Tetsutetsu & Itsuka Kendo vs. Mustard • Katsuki Bakugo, Shoto Todoroki & Fumikage Tokoyami vs. Moonfish • Ochaco Uraraka & Tsuyu Asui vs. Himiko Toga • Bakugo Escort Squad vs. Vanguard Action Squad)|